Light emitting diodes (LEDs) have found utility in a variety of applications from common light sources, such as flashlights and automotive headlights, to photonic interconnects for data transmission. An LED is a semiconductor device that spontaneously emits a narrow spectrum of light when electrically biased in the forward direction of a p-n junction. Light is created in, and released from, the p-n junction, which is more commonly referred to as the active layer.
LEDs generally include an n-type substrate with an active layer, a p-type layer, and an electrode attached to the p-type layer deposited on its surface. Current LEDs often utilize a double heterostructure, which includes an n-type contact layer, an n-type clad layer, an active layer, a p-type clad layer, and a p-type contact layer superimposed on a substrate. The use of double heterostructure LEDs provides increased efficiency by better confining carriers in the active layer.
The active layer of an LED is commonly doped with a donor impurity such as Si or Ge and/or an acceptor impurity such as Mn or Mg. Doping increases the emission power of the active layer, because more carriers, such as electrons and holes, are provided in the active layer, but doping also has negative effects on the performance of the LED. For instance, doping increases carrier lifetime and, thus, reduces the speed of the LED. Therefore, because doping trades power for speed, doped LED's have limited utility in applications that require high modulation speed, such as photonic interconnects for the transmission of data.
Attempts to improve the speed of LEDs include the use of quantum well structures in the active layer. A quantum well confines energy and increases light emission output when compared to conventional doped active layers due to the quantum size effect. However, conventional LEDs utilizing quantum wells do not provide sufficient power because they lack an adequate supply of carriers. As mentioned above, adding carriers, by doping for instance, reduces the speed of LEDs. Therefore, current LEDs are limited by either speed or power.
An additional drawback of conventional LEDs is that they are also hampered by poor efficiency. Efficiency is a two-fold problem. First, light production efficiency refers to the generation of light in the active layer. LEDs having doped active regions produce light slowly, while LEDs utilizing quantum wells produce light more quickly, but at less power, as mentioned above. Second, extraction efficiency refers to the amount of light extracted from the active layer after the light is generated and the speed at which the light is extracted. Extraction efficiency is a problem with all conventional LEDs because the semiconductor layer adjacent to the active layer reflects the light back into the active layer. Thus, the lack of overall efficiency of conventional LEDs practically limits their use in high speed applications, such as photonic interconnects.